Various structural alternations of gonadotropins and their receptors (R) contribute to the polymorphism of gonadotropin action.One reason is the microhetrogeneity of gonadotropins due to variations in the degree of their glycosylation.This alters the intrinsic bioactivity of gonadotropins, as reflected by changes in their bioactivity to immnoreactivity ratios in various physiological and clinical conditins.We have reassesses this phenomenon by improved in vitro bioassay and immunoassay methods, and it appears that the intrinsic bioactivity of gonadotropins, in particular of LH, is more constant than previously demonstrated.The second part of this chapter deals with a common polymorphism that was recemntly discovered in the gene of the LH beta-subunit.